1. Field of the Invention
The present invention relates to a solid oxide fuel cell (SOFC).
2. Description of the Related Art
An SOFC includes a fuel electrode for allowing a fuel gas to be reacted, an air electrode for allowing a gas containing oxygen to be reacted, and a solid electrolyte film provided between the fuel electrode and the air electrode (see, for example, Japanese Patent Application Laid-open No. 2010-3478). When a fuel gas (e.g., hydrogen gas) and a gas containing oxygen (e.g., air) are supplied to the fuel electrode and the air electrode of the SOFC at high temperature (e.g., 500 to 1,000° C.), respectively, chemical reactions represented by the following formulae (1) and (2) occur. Through the chemical reactions, a difference in electric potential is generated between the fuel electrode and the air electrode. The difference in electric potential is originated from oxygen conductivity of a solid electrolyte.(1/2).O2+2e−→O2− (at air electrode)  (1)H2+O2−→H2O+2e− (at fuel electrode)  (2)
In general, there may occur a phenomenon in which a reaction layer having large electric resistance is formed at the interface between the air electrode and the solid electrolyte film of the SOFC which operates at high temperature through a reaction of components for the air electrode and the solid electrolyte film. Specifically, for example, when lanthanum strontium cobalt ferrite (LSCF) is used as a material for the air electrode and yttria-stabilized zirconia (YSZ) is used as a material for the solid electrolyte film, it is known that a high-resistance layer (SrZrOx) is liable to be formed through a reaction of strontium (Sr) in LSCF and zirconium (Zr) in YSZ. Moreover, it is also known that a high-resistance layer (La2Zr2O7) is liable to be formed through a reaction of lanthanum (La) in LSCF and zirconium (Zr) in YSZ.
Formation of the high-resistance layer may be one of the causes for increasing interfacial resistance between the air electrode and the solid electrolyte film to decrease an output of the SOFC. Therefore, it is preferred to suppress the formation of the high-resistance layer. From this point of view, in the SOFC described in the above-mentioned literature, a porous reaction prevention film including ceria (e.g., gadolinium-doped ceria (GDC)) is provided between the solid electrolyte film and the air electrode. The porosity of the reaction prevention film is set to 10 to 80%.
In addition, in the SOFC according to the above-mentioned literature, a precipitate, which is formed through precipitation of a “component for the air electrode,” is entrapped (accumulated) in pores of the porous reaction prevention film. Thus, the “component for the air electrode,” which moves in the reaction prevention film through diffusion, is unlikely to reach the interface between the reaction prevention film and the solid electrolyte film so that the formation of the high-resistance layer is suppressed surely, according to the above-mentioned literature.